I was looking for the behavior of the Refresh on the underlying data, wondering if the there was a truncate parameter. According to the docs the atomic refresh has to do with how the transaction is handled and not how the data is removed from the table.
from the docs posted above it does not specifiy delete or truncate
The Datawarehouse Guide explains it.
For refresh using
DBMS_MVIEW.REFRESH, set the parameter
COMPLETErefresh, this will
TRUNCATEto delete existing rows in the materialized view, which is faster than a delete.
Tom Kyte is absolutely right as always. If you set atomic_refresh = false, Materialized view will be truncated and inserted with append hint. Only negative of this is that your data will not be available to user after truncate until your insert part is complete. So if you go with truncate option, you need to think about unavailability of data.